Coincidence circuit



Sept. g, 1952 R. E. BELL coINcIDENcE CIRCUIT 2 SHEETS-SHEET 1 Filed Oct.25, 1950 Sept- 9 '1952 R. E. B ELL 2,610,303

COINCIDENCE CIRCUIT Filed oct. 25, 195o 2 SHEETS-SHEET 2 R5 R5 '/65 L2l-1 'Y2 V5 YZ VOLTAGE WAVE FORM' v AT @man 0F V20/P V5.

NME-

"'F'.:| E=| "-'E| VOLTAGE v wAvEvFo/M i l l l n Ar PLATE 0F v2 0E V5.

vaLrAGE l, PULSE REFLECrED FROM 7 CABLE L2.

v NAVE/:ORM Ar JuNcr/o/v /4- 0;- cAeLEs L/,L2,L5.

Arry` Patented Sept.v 9, 195.2,

signor -to National Research Council, Ottawa, Ontario, Canada, a bodycorporate of Canada Application october'izs, 195.0, seriai No. 191,715`secunia/rc1.25o-207) The invention relates cuit. for determining .thetime 'correlation ,be-

tween pairs of pulses such as mayfoccur in .nucleari physics and cosmicray experiments.

In general, a dual coincidencecircuit.isa device which formsan-output'pulse when` a-pulse from each of twoseparatesources is fed to'itsinput within aninterval of. time lcalled the re' solving time of.the circuity (designated belovv as Ifithetwo input pulses are spaced intime by a greater interval than theresolvin'g. time of the circuit, nooutput pulse is formed. In manyl of the .uses'of a coincidencecircuit, adifficulty encountered .is .the .occurrence .of chance coincidencesduring thelresolving timeof the circuit between. pulses. which are only.accidently related in time. coincidences is' proportional tothe productof the resolving time and the. rates .of arrival fof"r the pulses'. fromthe two sources and, therefore, an

effective Way to keep the chance coincidence rate small compared withthe true'coincidence'rate is: to use a circuit having a short resolvingtime. Also, ,a/coincidencecircuit having a short resolving time isuseful tomeasure time .intervals .be-

tween. pulses. Vwhich follow each other atv short InA coincidencecircuits Vvknown prior to the present invention, the pulses, beforereaching the actual coincidence determining elements of .the circuits,were pre-selected to meet a' minimum pulse height requirement andsometimes were alsov pre-treated soV that they conformed to astandardpulse height and shape. If the input pulses were not uniform insize and shape, the

to .a dual coincidence Vcir- The rate of occurrence of. chance.

ynist input connected to the output of the first cidencecircuits hadsuch a long resolving time" that they were of little'or-no Value linexperiments' andmeasurements requiring a very short resolving time, forexample, as short as 10-9 second.

A dualcoincidence circuit having avery short resolving timeis madepossible by the present inventors discovery that pulse heightselectioncanbe made after coincidence determination.-

According to the present invention, adual coin-* cidence circuit havinginputs for electrical pulses' fromv tw'osources comprises,meansconnected. to each of the inputs for limiting thev a'n'iplitudev 'of thepulsesreceived at each' of the inputs toa pre'- determined value, apulse terminating meansfor' terminating each pulse from each of theitw'o sources after a predetermined lifetime, adual coincidence mixermeans'arranged-to bebi'asedtd supply anI output pulse onlyforlcoincidentinput pulses greater than the predetermined.va1ue,fthepulse terminating means having its input connected to the outputs of theamplitude limitingmeans .and having its output connected-tof-V the inputofthe mixer means, .a first pulse amplitude analyzer, the mixer meanshaving its output corinected tothe input of the first pulse amplitudeanalyzer, a triple coincidence circuit having a pulse amplitudeanalyzer, second and third pulse amplitude analyzers having their inputsrespectively connected to the outputs of lthe `one and of` the other ofthe sources and having their outputs respectively connected to the.second 'and third inputs ofthe triple coincidencev circuit, and pulseindicating means connected to'theoutp'ut' ofthe triplecoincidencecircuit.' l 11E, It is preferred, according to the.presentinvenI-l tionyto use a predetermined lengthof shortcir-a. cuitedtransmission linefas the .pulse terminate.l

f ing means. Predetermined lengthsof transmis-if."

process, of pre-selectingand pre-treating, introduced delays of varyingamounts .in the arrivals Y ofthe standardized pulses at thecoincidencedei termining element of the circuit. The variation inthe-amounts of the;delaysmdepended'onthe differences in the originalsizes ofthepulses, vand itzwas a requirement ofthe prior art coincidencecircuitsto have a resolving time great enough to accommodatethe-variations in size of the incomingpul'ses'. `The` result was thatpriorfartcoinsion line can also be' used' With advantaglto transmit`.the pulses to the pulse terminating means A'since the 'le'n'gth's.used can be predeterminedxto compensate for unequal delaysfto which'Figure 1 is a schematic block diagram of a cirf cuit embodying theinvention,

Fig. 2 is a schematic diagram of part of the circuit shown in Figure 1and showing amodication of the circuit shown in Figure 1,

Figure 3 is a curve showing the waveform at the grid of V2 or V3 ofFigures 1 and 2 which re sult from an incoming pulse,

Figure 4 is a curve showing the Waveform at theplate of V2 or V3 ofFiguresl and2 resulting f frein anfincomine pulse, and

Figure 5,-is a curve lshowing the YWaveform at the .anode of- V5 ofFigures 1 and 2 resulting from a pulse as shown in Figure 4.

A pulse jcoincidence counter, including a, circuit according.totheinvention is shown in Figure 1 and comprises a pair ofscintillation counters having` phosphore .9 and photomultiplier tubesVLand V4whose anodes I0 are connected respectively through couplingcondensers CI to the controlgridslof pentode type vacuum tubes V2 andV3.; The photomultiplier tubes VI and V4 have grounded cathodes II andhave, dynodes I2 interconnected by resistors `RI having a resistanceqflimegohm.; The anodes I are respectively connectedthrough 10,000 -ohmload resistors R2 to a 124.000 volt supply. The control grids of thetubesViZ and V3 are connected to ground by grid condensers R3, thesuppressor grids and the cathodes being directly grounded. The` screengrids. ofthe tubes V2 and V3 are kept at a suitable positive potentialby connections through resistors R4 to a +150 volt supply and arebypassed 15.0,. ground b y condensers. C2. The plates of the tubesIVZand V3. are supplied withapositivepotentialfrom the` +150 volt supplythrough 100 ohm load-resistors R5 and are connected through couplingcondensers C3 te coaxial transmission lines` LI .and-L3. Thetransmission lines LI and L3. (10.0 ohm. characteristic` impedance) meetat a commonvelectrical junction I4 with a short circuited .transmissionline L2 (50 ohmcharacteristie impedance) The common electrical june?tion I4 of the transmission lines is connected to the` anode. .of .adiode V5 which may be, for eX- ample,an electronic. vacuum tube or agermanium crystal. The outer shieldof each of the transmission lines LI,L2 and .L3 is grounded.

f .'The.. cathode of the diode V5 is connected through'a 10,000 ohmresistor R6 to the variable tap of a potentiometer R1 which has its highpo. tential end taken to a +2 volt supply and its low potential: endtaken to ground. The cathode of the. diode V5. is connected through acoupling condenser C4 to. the input of. an amplifier AI which may .beef`conventional design for the amplifica.- tion :of pulses. For example,the amplifier AI may have a gain ofabout 20,000 and a band Width of'`about 2 megacycles. The output of an amplifier AI connects. to the inputof a pulse amplitude analyzer DI of conventional design. The output ofthe pulse amplitude analyzer DI is connected to one of theinputs of atriple coinf cidence v circuit I5.

A suitable design for the amplifier AI and the pulse amplitude analyzerDI is described in the article entitled A General Purpose LinearAmplifier by W. H. Jordan and P. R. Bell in volume 18, number 10,October 19, 1947, issue of The Review of Scientific Instruments at pages703 to 705. The pulse amplitude analyzer may be a pulse heightdiscriminator or a kicksorten The triple coincidenceltype of circuit isWell lknown and `may be of-the type described in the April 26, 1930issue of Nature, page 636, by Bruno Rossi.

The second input of the triple coincidence circuit I5 is connected tothe anode I0 of the photomultiplier. tube VI through the couplingcondenser CI, a vresistor R8, an amplifier A2 and a pulse amplitudevanalyzer D2. The third input of a triple coincidence circuit I5 isconnected t0 the anode I0 of the photomultiplier tube V4 through thecoupling condenser CI, the resistor R8, an amplifier A3 and a pulseamplitude analyzer D3. The amplifiers A2'and A3` areof similar design'tothat of theamplifier AI and the pulse amplitude analyzers D2y and D3are of similar de,- sign to that of the pulse amplitude analyzer DI. Theinput of the` triple coincidence circuit I5 connected to the pulse,amplitude analyzer -DI has a sealer SI of conventional/design connectedto it. Each ofthe secondvand third inputs ofthe triple coincidencecircuit I5 has a ratemeter or sealer S2 Vand S3 connected to it. Theoutput on the triplecoincidence circuit yI'5is connected toa sealer S4.The -ratemeters or scalers SI, S2, S3 and S4 are of ,conventionaldesign.

the followingdescription of the operation of the circuitof Figure 1, it-isv assumed that nuclear radiations for example, alpha, beta andgammarays, emitted from a sample ofradioactive material are to be counted.The phosphors- 9 Vfor the photomultiplier tubes VI and V4 are placed,relative to the sample of radioactive material, in position such thatthe nuclear radiations emitted will reach `the phosphor 9 of each of thetubes substantially simultaneously. Upon a nuclear radiation excitingthephosphors 9 of the tubes VI and V4, photons are collected on thecathodes I l of the tubes VI and V4v causing thecathodes II te emitelectrons. As is Well known, emission of electrons from the cathode of aphotomultiplier tube .results in the forming Lof .an avalancheofelectrons directed towardt'he; anode of the tube. In apparatus as shownin'Figure 1, 'the avalanches of. electrons reaching the anodes ofthetubes.

VI and ^V4 .form output voltage. pulses which are applied to the controlgrids of the tubes V2 and V3. Thewaveforms ofthe pulses supplied tothegrids ofthe tubes V2 and V3 are as indicated in Figure 3. The anodecurrents in the tubes V2 and V2 arey cut on". due to pulses applied totheir control grids thus limiting the amplitude of the pulsesV to apredetermined Value .as shownl in Figure 4. The anode .currents in thetubes V2 and V3 recover after an interval of rtime -deter. mined by thetime constant of the resistor R2 and the interelectrode capacities ofthetubes VI and V2. If, for example, the resistor -RI had a re-` sistanceof 10,000 ohms and the capacity to ground During this time V2 is cutoif, and anyl spurious pulses following the main lpulse would notkaffect the operation of the circuit. Y

If itis supposed that. the transmission line LI (or L3) is of infinitelength, the waveform at the.l

plate ofthe tube V2, (or V3).y wouldbe as shown,v

in. Fieurel. assuming that the. plate .circuit ott-11e.

gemene tube V2 (or V3) had af very short time constant.- However,theitransmis'sion lines .LI1and'fL3 are terminated at the junction I4andfif each o'f these transmission lines has a lohmcharacteristicimpedance, and the shortcircuitedv transmission 1ine-L2has a 50 ohmcharacteristic! impedance, the transmission lines LI and'L3 being.matched at the .plates of the tubesf.=V2.fand VV3rby there-r sistors R(100 ohms) ,no unwanted multiple :re-.- flection'swould foccur.lHoWeverp-the short circuitedf transmission'L -linei L2 reiiects thepulse transmitted to itfrom the transmission'line .LI (or L3) after anelapsed.v time of 2p seconds,.where p is the pulse transit time of the.transmission line L2. `The result ofthe reected pulse-fromthe` shortcircuited transmission line L2 is indicated.

in Figure 5 in which it is 4shov'vnfthatfatpart. of a pulse f-incidentllon .the junction: 'I4' `of the. transmission lines LI,L2 and L3 iscancelled". by the pulse reflectedA from` the shortv circuited'trans.-

mission line L2. The result is. an occurrence. of

a pulse having a duration of 2p secondsratthe.

. the .pulse formed issmeared `out tin lengthand maynever reach. itsmaximum amplitude. The

maximum vamplitude of thezpulse formedat thev junction I4 is the productofi lthe.v paralleLfimpedance of the transmissionlines LI; L2and L3bythe standing plate current of the tube V2 (or V3)'. For example, [thestanding plate current of the tube V2 '(or V3) may be 10 ma. and atypical value of the parallel impedance of the transmission lines` LI,L2 and L3 is 25 ohms giving the pulse voltage as A volt.

Under ideal circumstances, with the bias ofthe diode V5 set at Mi volt,'no pulse would 'reach the input of'the ampliiier AItTIn practicesmallpulses wouldbe transmitted through the .capacity of the diode V5 andwould Vreach rthe input of the amplifier A2 with Widely distributedamplitude. Such pulses are removed from the input of the amplier A2 bysetting the pulse amplitude analyzer D2 to a bias level E which is ljustlarge enoughv to reject them all. If VI and V4 are each transmittingpulses, two pulses which Occur within the resolving time of theamplifier AI but not within 12p seconds of each other, will produce amammum possible output from AI of 2E due tothe capacity-transmissioneiect, since AI integrates the pulses. If, however, the pulses occurwithin i213 seconds of each other, the maximum pulse amplitude at thejunction I4 would be greater than 1A volt and the diode V5 would conductthe .top part of the combined pulse. Therefore, amplifier A2 would havea large output, equal to many times E, and so the pulse amplitudeanalyzer DI should be set to reject all output pulses less than 2E. Itis important that the pulse amplitude analyzer DI be not set ata leveljust greater than E (in which case the ampli--` fier AI. would at leastin part determine theresolving time) but be set at a level greater than2E (in which case the diode V5 alone is the nonlinear coincidenceelement). The output of the pulse amplitude analyzer DI consists ofunserecorded by the ampliers A2 and A3, the pulse amplitudey 65analyzers.v D2" xand D3 andlthe scalers 'S2 andfSi Th'eampli'ers A2 andA3 accept vuntreated pulses fromthephotomultiplier tubes :VIriand V4,land thepulsef amplitude analyzers D2 .and D3lselect `i onlythose pulseslarger 4than a predetermined value. -The triple coincidence unit I5selects, out of. all the fast coincidences from the pulse ampli# tude4analyzenzD I only thosel fast coincidences which :have been.v caused by:,individual `'pulses'. larger thanV the sizes selectedby D2.and D3.or,:in the case of using kicksorters; within the range se-v lected by thekicksorters.. The individual. rates of pulse arrival can be determinedfrom the read--1V Vings ofV the Sealers- S2 ands3,;and thecorresf,

" ponding coincidence Vrate from the reading of the scalerS4.; Theresolving time 210 of the coincidence circuit is much iniiuenced by thebias settings Abut should be about equalto 4p seconds. Initypcalexperimental cases: ,l v A. 'jg- ;z

rise-time ofthe pulse isdetermned chiefly bythe decay time of the lightfromthe phosphor. VWith the presentapparatus the resolving time canine,made to be less thanv 1A the rise `time offthe pulse. The resolutioncurve is of course, observedby measuring coincidence rate :as`aiiunction of the lengths of the transmission lines LI and L3. using aradioactive source known to contain promptly coincident radiations. Thecoincidence rate is plotted as a function of delay timer for the twocounters, yielding a bell-shaped curve of width noted above. Theresolving time can also be computed from the observed chance coincidencerate and the two individual counting rates: the chance coincidence rateis the coincidence rate observed when there is so much delay inserted inone or other of the two counter cables that no true coincidences canoccur.

'I'he transmission lines LI and L3 are not necessary under circumstancesin which the` photomultiplier tubes VI and V4 are matched in regard to`transit time and the phosphors 9 are located at equal distances from theradioactive source. The connections between the tubes V2, V3 and thediode V5 are shown in Figure 2 with the transmission lines LI and L3omitted. However, in most cases, the transmission lines LI and L3 arerequired to compensate for unequal delays in the arrival of the vpulsesat the tubes V2 and V3, or to obtain a measure of the time of night of aparticle from a radioactive source, or

to measure' a short radioactive lifetime.

7 nulsertermnating, meansfforif. ter'ihxating; ueach puise-from :eachiofegsaid sources .z after gagipredef-s termined j 1ifetime,:,dua'1coincidence `mixer vmeans arranged tozzbe biasedto tsupplyrany `output:pulse only' ,fforccoincidenti input pulsesewhoseicombined `valueisz-greater, than apredetermined magnitude,

said pulse a' terminating azmeans :xhavingi: its iinput connected to the:outputs of theamplitudexlimite ingrmeansfandhaving its'foutputconnectedtovT the inputo'f A.said mixer means ,fa rst-fpulse amplitude analyzer,sai'drsmxer means :having'its output connected rito the input. of thenrst pulsezamplitudef analyzer, :atriple'coincidence: circuit havingaffrstiinput .connected to the 'output `of the first pulsel amplitudeanalyzer, second 'and third l'pulse amplitude analyzers having theirinputs respectively connected to the 'outputs of 1 'thefonef'andofftherother of said sources-andhaving theirout-V puts Y 'respectively`connected to the #second v'and third inputs of the triple coincidencecircuit, -and pulse indicating means connected to the outputofthe-triple coincidence circuit.

2. A dual coincidence circuit as dened in claim 1 comprising a rst and asecond means for delaying the electrical pulses for a predeterminedinterval of time, the viirst-and the second delaying means beingconnected to delay, respectively,V the transmission* of pulses from onesourcefgand from the other vsource 1to1Y the Avpulse' terminating means.

3. A Adualcoincidence Icircuit vas defined in claim 2 in rWhich thepulse terminating means is a transmission line of predetermined lengthshort circuitedat `one end and open circuited Vat the-.other end,: the:open `eircuited end serving as an input :and -anvoutput "for :saidipulse :termi-i natngf-meansgand the; first 'and' second delaying meansare: eachvar predetermined :length of -transmission Aline conneetion, Arespectively, ,the =iout putsfof the oneandzof the otherof'zthe-ampltudelimitingsmeanszto theopen circuitedend .of .the shortcircuited.transmission line.;Y

A :dual coincidence. eircuitzyassdenedzrin Cil claim -3.inzwhieh.thezsources of' electrialgmulses are` '.ph'otomultiplierl :tubesarranged tobe Aoper-jated f'atf high enough svoltagesfto jproduce; -pu15havingeen-ma'gnitudefol at leasta volt-when said tubes; are.exeitedhy.seintillations.v

5;:A dualncoincidence circuit .as ydefnedfin clainxzzcomprisinejpulsecount indicatingl means vconnected:topeachzinputof .the triple:coincidence x6: Arzdual coincidence circuit tas defined claim 1:,1which`Ithe pulse .terminating meansiisy atransmission `line-cf predeterminedlength short circuitedziat-z one v:end fand open circuited at 'f theothereendgithezopen circuited end serving gas.. an input :rand sanioutputifor the Ypulse `terminating lrleazm.,`

'1.-v A= dua,1i coincidence-i circuit as' defined V; in.

: RoBEiemfE-.BEIL

fREFERENcEsoCIrE-D The fol-lowing references Aare of f vrecord" in :thefile of this patent:

